Development of polarization modulator using MXene thin film

In this work, polarization modulator utilizing MXene material, namely Nb2C is demonstrated. S band signal is injected into Nb2C thin film and is modulated by 1400 nm laser diode. A total of 39.81° of polarization rotation is attained when the pump power is increased to 223 mW. The rotation of light is due to thermo-optic effect. The efficiency of polarization modulator is calculated at 0.1974°/mW.

In this work, Niobium Carbide (Nb 2 C), a family member of MXene, is processed into a thin film and integrated into all-optical system to function as polarization modulator in S-band region. The fundamental concept of this work is to induce thermo-optic effect on Nb 2 C thin film by pumping high intensity continuous wave (CW) laser. The accumulation of heat in Nb 2 C thin film alters the refractive index of itself, and thus modulate the polarization state of the signal light that propagate through the thin film. Throughout the pump power range of 0-223 mW, the proposed polarization modulator has achieved a rotation of 39.81°, which gives the modulation resolution of 0.1974°/mW. To the best of authors knowledge, this is the first all-optical polarization modulation in S-band region using MXene as modulator.

Fabrication and characterization of Nb 2 C
Fabrication of Nb 2 C/PVA film. The solution casting technique was utilized to fabricate the Nb 2 C/PVA film which follows the same procedure as reported in our previous work 28 . Initially, 50 mg of Nb 2 C powder was mixed into 10 mL of the IPA solution using a Hielscher UP200Ht Handheld Ultrasonic Homogenizer probetype sonicator. The mixture was undergoing a sonication process for 4 h to obtain a homogenous suspension. The sonicator was set at 5 s on and off pulse sequence under the operating power of 80 W. In the next step, the suspension was centrifuged at 4000 rpm for 10 min to remove the undissolved powder and collecting the supernatant for further used. In order to fabricate the film, 5 mL of Nb 2 C solution was added into a glass beaker which consist of 5 mL of the PVA solution (10 mg/mL concentration) under constant stirring at 70 °C. After 1 h of stirring, the obtained sample was dried in an oven at the temperature of 60 °C for several hours. At last, the fabricated Nb 2 C/PVA film was obtained by peeling off the film from the petri dish and kept in a clean container for storage until further used as SA material. Owing to the low absorption loss in S-band region, the PVA is chosen as to form the thin film with Nb 2 C in this work. The Nb 2 C MXene powder was received from 2D Semiconductors and the polyvinyl alcohol (PVA) powder (MW ~ 31,000) and isopropyl alcohol (IPA) (~ 99.7%) solution were purchased from Sigma Aldrich. All reagents were used as received without purification.
Characterization of Nb 2 C MXene. The crystalline structure and phase of Nb 2 C MXene was examined by a Malvern Panalytical Empyrean X-ray Diffraction (XRD) operated using Cu Kα radiation. Figure 1 shows the XRD pattern for Nb 2 C MXene scanned from the 2θ of 5°-70°. A broadened diffraction peak located at about 9.04° was associated to the (0 0 2) plane of the Nb 2 C MXene. It has been reported that this peak was arise by cause of the interruption on the Nb-Al bond which lead to the increment of the interlayer spacing. The appearance of this diffraction peak has evident the successful of removing the Al layers upon the etching process and hence forming the Nb 2 C MXene 29 . Moreover, three more diffraction peaks can be observed at 33.68°, 38.01°, and 60.00° which ascribed to the (1 0 0), (1 0 1), and (1 1 0) plane of the hexagonal Nb 2 C MXene (JCPDS file 00-015-0127) 30 . The absence of other impurity peak indicates the high purity of the Nb 2 C MXene sample.
The Raman spectrum was obtained by an inVia Renishaw Raman spectrometer with a 514 nm laser as the excitation source and the result was presented in Fig. 2. According to the obtained result, a peak located at 261 cm −1 was observed which associated to the A 1g mode that arises due to the symmetrical out-of-plane vibrations of Nb and C atoms 31 . Besides that, two more characteristics peaks were appeared at the Raman shift of 1338 and 1597 cm −1 that can be indexed to the D and G bands of carbon species, respectively, in which carbon is one of the main constituent for Nb 2 C MXene sample. The D band refers to the carbon disordered structure of www.nature.com/scientificreports/ the sp 3 hybrid carbon meanwhile the G band is assigns to the graphitic band or sp 2 carbon atom 32,33 . It is worth noting that the remaining peaks located at 520, 634, and 984 cm −1 corresponds to the characteristic peaks of silicon (Si) substrate 34 .
The high resolution transmission electron microscope (JEM 2100-F HRTEM) was utilized to investigate the morphology of the Nb 2 C MXene. Initially, the Nb 2 C solution was diluted by using ethanol as the solvent and the solution was sonicated for 20 min before drop casted onto the copper mesh for testing. Figure 3 depicts the HRTEM images of Nb 2 C captured at different magnification. As can be seen in Fig. 3a, Nb 2 C exhibits the layered feature with irregular shapes and sizes and the self-stacking occurs between the nanosheet layers. The light grey color indicates the thin area of the sample whereas the black color implies the much thicker area. Based on the HRTEM image of Nb 2 C obtained at high magnification, a well-defined hexagonal lattice can be noticed in Fig. 3b, which signifies the high crystallinity of Nb 2 C MXene. The interlayer spacing of Nb 2 C MXene was measured to be about 1.8 nm, thus verifies that the Nb 2 C samples consist of multiple layers. The thickness of the Nb 2 C-PVA film was measured using a Dektak D150 surface profiler after the produced film was placed on a glass slide. The Nb 2 C-PVA film was found to have a thickness of 52.5 μm as shown in Fig. 3c. The insertion loss of Nb 2 C/PVA thin film was measured about 3.2 dB at wavelength of 1500 nm. Figure 4 shows the proposed setup for polarization modulator. A 1400 nm laser diode is used as a pump source. Santec TLS 550 tunable laser source (TLS) provides an input signal to the polarization modulator. TLS is set to 1500 nm and 0 dBm. A polarization controller (PC) is connected to TLS where it is functioning to rotate the light from TLS into polarized light. The 1400/1500 Wavelength Division Multiplexer (WDM-1) multiplexes both signals from laser diode and TLS. Nb 2 C thin film is cut into 2 × 2 mm and sandwiched in between two fiber ferrules. Another 1400/1500 (WDM-2) is located after Nb 2 C thin film to guide the input signal into Thorlab PAX1000 polarimeter for analysis.

Results and discussion
Tunable Laser Source (TLS) serves as an input signal to the polarization modulator. It is fixed at wavelength of 1500 nm and output power of 0 dBm. The orientation of PC is adjusted to change the TLS signal to a linearly polarized input signal, with − 11.94° azimuth and ellipticity of 0.13°. The linearly polarized input signal is as shown in Fig. 5. The measured output power is − 4.4 dBm. This indicates that the total loss of − 4.4 dBm is contributed by the thin film, connector, WDM and light polarization loss. The 1400 nm pump power is gradually increased up to the maximum available pump power at 223 mW. Figure 5 describes the polarization state rotation of light observed at polarimeter with the increment of pump power. The blue indicator on the spherical plot illustrates the polarization state of signal in 3 Dimension (3D) view. The blue indicator is moving in a counterclockwise direction with the increment of pump power. At 87.9 mW pump power, light azimuth is recorded at 0.08°. When the pump power is further raised to 119 mW, the indicator crosses to 7.67° and finally to 27.87° at pump power of 223 mW. As can be seen from the Fig. 5a-f, the indicator moves steadily near the horizontal axis, throughout the increment of pump power, signifying there is a continuous azimuth rotation. Figure 6 provides a better illustration on the rotation of light azimuth and ellipticity. The initial polarized light from TLS is represented by a straight line, tilted at an azimuth angle of − 11.94° and ellipticity of 0.13°. The light experienced counterclockwise rotation throughout the increment of pump power. At 87.9 mW, the light polarization is horizontally aligned with azimuth angle 0.08° while its ellipticity of 0.06°. At 223 mW, the measured azimuth and ellipticity is 27.87° and 3.77°. 564 . From the linear regression trend, there is no indication that the polarization rotation experience saturation with the increase of pump power, so the light azimuth can be further rotated if higher pump power is employed. When MXene absorbs continuous wave (CW) pump laser, it induces oscillation of electron from occupied state to unoccupied state and creates hot electrons that leads to thermal charge carrier distribution. Then, the hot electron cools down through transferring energy to the lattice phonon which results in a temperature increase 35,36 The heat generated from the increment of pump power propagates through Nb 2 C thin film and modifies its refractive index due thermo-optic effect, which can be expressed by www.nature.com/scientificreports/ where n 0 is the refractive index at temperature T 0 , dn dT is the thermo-optic coefficient and T is the change rate of temperature. As a result, the propagation path of light changes, which leads to change of phase, and subsequently modulates the polarization state of signal 37,38 .
The change of light ellipticity and output power with increment of pump power are illustrated in Fig. 8. No particular trend is discovered on the light ellipticity, but it fluctuates between 0.06° and 4.08° as the pump power is increased from 0 to 223 mW. On the other hand, the measured output power is increasing with pump power. The output power increased from − 4.64 to − 1.27 dBm throughout the increment of pump power. The increase of pump power feeding the absorption of the Nb 2 C thin film. As a result, the absorption of the Nb 2 C thin film      www.nature.com/scientificreports/ state. It is worth to note that this work reveals higher modulation resolution compared to 16 and the fluctuation of ellipticity is smaller (4.08°) compared to earlier work where the reported fluctuation was 10°. This work has proven the potential of MXene thin film to function as a linear polarization modulator. The proposed work is an alternative solution to compensate light polarization in an optical network by simply controlling the pump power. This set up can be installed along optical fiber to achieve acceptable polarization dispersion and bit error rate (BER). Therefore, the realization of long distance and high-speed optical network for future communication is feasible with this solution.

Data availability
All data generated or analysed during this study are included in this published article [and its supplementary information files].